Resistance coupled amplifier circuits



06L 1940- E. w. HEROLD RESISTANCE COUPLED AMPLIFIER CIRCUITS Filed June 16, 1937 pas/WE NEGATIVE mmcon/oucr/wcs rmr0, oz/m/r 0; war r05 3 l Hg.1

SIG/VAL ENERGY 0 L m RE m M m VO V0 .T. NR T 2 1? m Y B Patented Oct. 8, 1940 STATES PTE RE SI STANCE COUPLED AMPLIFIER CIRCUITS Delaware Application June 16, 1937, Serial No. 148,461

6 Claims.

My present invention relates to resistancecoupled amplifiers, and more particularly to novel and improved arrangements for neutralizing the input admittance of such amplifiers thereby to improve the frequency response range of the latter.

Neutralization of the input admittance of an amplifier tube has been frequently accomplished by the use of feedback from the anode. A particular type of neutralization is that in which the input admittance is reduced to a constant positive value over a wide band of frequencies. In resistance-coupled amplifiers it has been proposed to secure input admittance neutralization by a two-tube, cascaded, resistance-coupled amplifier with a feedback capacitance from the sec-- ond tube anode to the first tube grid. Heretofore, attempts to accomplish this have been accompanied by clifiiculties: failure to reduce input admittance to the desired value; self-oscillation of the circuit; and generally critical operation, have discouraged repeated efforts to utilize this type of neutralization.

It may be stated as one of the main objects of my present invention to provide a two-tube, resistance-coupled amplifier stage wherein one of the tubes is of the negative transconductance type and has a feedback circuit from its anode to the grid of the other tube, a positive transconductance tube, thereby to secure input admittance neutralization.

Another important object of my invention is to provide an amplifier having a resistance coupling to its input circuit, and wherein the amplifier includes a negative transconductance section having a feedback connection to the input electrode of the amplifier to neutralize the amplifier input admittance.

Another object of the invention is to provide resistance-coupled amplifiers, whether used for wide band amplification or so-called resistance tuning, with tubes which each include positive and negative transconductance sections; the negative transconductance section in each tube acting to neutralize the input admittance thereof.

Still other objects are generally to improve the efiiciency and stability of resistance-coupled amplifiers employing input admittance neutralization, and more especially to provide a highly economical type of tube functioning both as an amplifier and as a negative transconductance device for amplifier input admittance neutralization.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims; the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically several circuit organizations whereby my invention may be car- 5 ried into effect.

In the drawing:

Fig. l is a schematic representation of a resistance-coupled amplifier circuit employing the invention, 10

Fig. 2 shows the manner of applying the invention to a resistance-tuning amplifier stage.

Referring now to the accompanying drawing, wherein like reference characters in the different figures designate similar circuit elements, there is shown in Fig. 1 a resistance-coupled amplifier stage which generally comprises a source of signal energy I, a tube 2 which includes positive and negative transconductance sections, a load resistor 3 and an input resistor 4. Resistancecoupled amplifiers are often used in high frequency communication systems, including those suitable for television. The amplifier network, in most cases, is desired to have a substantially flat frequency characteristic from a low frequency to a comparatively high one. At the higher fre quencies the unavoidable shun-t capacitances cause a reduction in the impedance of the coupling elements, and so reduce the amplification. Compensation for the undesirable capacitances to secure uniform gain over a wide band of frequencies, has been accomplished by special equalization circuits; feedback circuits have, also, been proposed to neutralize the admittance of the coupling elements.

By utilizing a tube with a negative transconductance, it is found possible to neutralize an admittance by feedback in a much more satis factory way than by feedback over more than one stage of amplification. The effective input admittance of a negative transconductance tube, with an appropriate feedback admittance from its anode to its control electrode, is negative. By connecting such a tube across an admittance which is inherently positive, such as the coupling resistance of a resistance-coupled amplifier, non-- tralization of the positive admittance by the negative is obtained. The result is an increased amplification over the range of frequencies for which 50 the neutralization is effective.

A particularly advantageous method of accomplishing the admittance neutralization is secured when there is employed a tube of the type designated by numeral 2 in Fig. 1. This type of tube 5 is provided with positive and negative transconductance sections; the positive section is employed as the signal amplifier following the input circuit whose admittance is to be neutralized,

while the negative section is used to produce the neutralization. There has been disclosed in my application Serial No. 737,692, filed July 31, 1934, U. S. Patent No. 2,139,366, December 6, 1938, a tube having electrodes arranged to provide positive and negative transconductance sections; such a tube is schematically represented in Fig. l by the numeral 2.

The tube 2 will be generally described, since the details thereof are disclosed in the aforesaid application. Briefly, the cathode 5 is successively surrounded by the inner screen grid 6, the control grid 1, the outer screen grid 8 and the plate 9. The numeral N1 denotes the anode of the negative transconductance section; electrode IE] is physically a pair of electrically connected slats, or plates, mounted on opposite sides of, and edgewise to, cathode 5. The pair of plates are disposed between the cathode 5 and inner screen grid 6. The operation of the tube is dependent upon the control of the electron flow from the cathode to the inner and outer anodesv With a given voltage applied to the control grid 1, the electron flow from the cathode 5 will divide between the inner anode l0 and the outer anode 9. As the control grid l becomes increasingly negative more of the electrons are turned back to the anode l0 thereby increasing the current flow through the circuit thereof.

If the control grid I is made suiiiciently positive substantially all of the electrons will pass through the control grid to the outer anode 8; the current through the inner anode circuit decreasing substantially to zero. The grid 6 serves both as a space charge grid, and as an inner screen grid. The control grid 1, being screened from both inner and outer anodes, has low capacitance to both of them. The tube has a more symmetrical characteristic with respect to the transconductance, and the inner and outer anode currents, than the usual negative transconductance tube. V

The source I is coupled across the input r sistor d through the usual coupling condenser t. A negative bias source H connected between the grounded cathode 5 and resistor l, maintains the control grid 1 at the desired amplification bias. The current source I? maintains the screen grids and anodes at proper positive potentials. The screens 6 and 8 are connected to a common point on the source [2; the anode 9 is connected to the positive terminal of source I2 through load resistor 3. The anode i0 is connected to the same terminal through resistor I 3. The control grid to outer anode section (positive transconductance) provides a normal resistance-coupled amplifier stage. The completely independent resistor l3, in the inner anode circuit, serves to provide the neutralizing voltage; the latter being fed back to the control grid 1 through condenser Cm.

It will now be seen that the control grid to inner anode section is used to produce a negative admittance effect in the control grid circuit through a coupling condenser thereby lowering the total admittance. By making use of the outer anode to feed into a succeeding stage, a series of cascaded stages may be used with a separately controlled 'and independent feedback arrangement in each stage. It is quite possible to obtain good neutralization with a low value of resistance in the inner anode circuit thereby resistance of the section 1-).

minimizing the harmful efiects of shunt capacitance in this circuit. By using a very much higher resistance in the outer anode circuit the output can be taken from thereacross with a consequently higher stage gain, and without in any way affecting the neutralizing operation.

The present invention may, also, be applied to a circuit employing so-called resistance tuning. In the latter type of circuit, tuning is accomplished by obtaining balances between negative and positive resistance. A fixed inductor and fixed esistor are combined with a fixed capacitor and a fixed negative resistance in such a way that the combination behaves as a negative resistance, the Value of which is greatly dependent upon frequency. The combination is opposed by a positive resistance so that at one particular frequency, which depends upon the value assigned to the positive resistance, the total resistance is zero if they are in series, or infinite if they are in parallel. Tuning is, therefore, carried out by varying the value of the positive resistance.

In Fig. 2 there is shown such a resistance tuning circuit making use of the parallel connection; the signal source is coupled across the network comprising fixed inductor L, shunted by fixed resistor R, in series with variable resistor 2G. The total effective shunt capacitance is shown as Co whose connections are dotted as this capacitance may be wholly inherent in the tube and circuit elements used. The tube 2 has its control grid connected to the high alternating potential side of coil L; the source H provides the desired bias for the control grid. The outer anode 9 is connected to the positive terminal of source it through resistors R1 and 253, coil L1 shunting resistor R1. The potential developed across the output load is transmitted through coupling elements 30 to the next amplifier. It will be understood that several similar stages can be cascaded in this way; the description given in connection with tube 2 applying as well to the succeeding stages.

The inner anode I0 is connected to the positive terminal of source I 2 through resistor 2i. The condenser C'm is connected between the anode ill and grid 1'. The control grid to inner anode section of the tube provides the negative resistance and Co, B and L provide the other fixed circuit element referred to. The resistor 2!] opposes the negative resistance provided by the combination of RLC'o and the negative Varying resistor 20, and also resistor 20', results in tuning change. The value L=R Co should be chosen and the negative resistance in the grid circuit should be kept slightly greater than R. Tuning is varied from zero frequency to a frequency determined by the L Co product by altering resistor 20 between the limits of R and zero. The negative transconductance section of tube 2 provides the necessary negative resistance; no extra tube is necessary. The negative resistance may be adjusted by variation of Cm; the internal anode load; or by one or all of the voltages applied to the tube. The resistors 26 and 2!) may be simultaneously Varied.

While I have indicated and described several systems for carrying my invention into effect, it

will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended. claims.

What I claim is:

1. In combination with a tube having a positive transconductance section and a negative transconductance section, a wave input circuit connected to the input electrodes of the positive section, a wave output circuit connected to the output electrode of the latter, a load element independent of said output circuit connected to the output electrode of the negative section, said load consisting of a resistor Whose resistance value is relatively low with respect to the resistance value of said output circuit, and means including a capacity element external of said tube for impressing the potential developed across the load element upon the input electrodes of the positive section.

2. In combination with a tube having a positive transconductance section and a negative transconductance section, a Wave input circuit connected to the input electrodes of the positive section, a Wave output circuit connected to the output electrode of the latter, a load element independent of said output circuit connected to the output electrode of the negative section, said load consisting of a resistor whose resistance value is relatively low with respect to the resistance value of said output circuit, means including a capacity element external of said tube for impressing the potential developed across the load element upon the input electrodes of the positive section, and the input electrodes of the positive section being the input electrodes of the negative section.

3. In combination with a tube having a positive transconductance section and a negative transconductance section, a wave input circuit connected to the input electrodes of the positive section, a wave output circuit connected to the output electrode of the latter, a load element independent of said output circuit connected to the output electrode of the negative section, said load element consisting of a pure resistance of relatively low value, and means including a capacity element external of said tube for impressing the potential developed across the load element upon the input electrodes of the positive section, said input circuit comprising a variable positive resistance network, and said negative section producing a negative resistance effeet between said input electrodes.

4. In combination with a tube having at least a cathode, control grid, output electrode and an auxiliary electrode positioned to receive more electrons when the control grid becomes negative, a wave input circuit coupled to the control grid and cathode, a wave output circuit coupled to the output electrode, a load circuit independent of the output circuit connected to the auxiliary electrode, said load consisting of a resistor whose resistance value is relatively low with respect to the resistance value of said output circuit, and a reactive coupling external of said tube connected between the auxiliary electrode and control grid for impressing on the control grid wave voltage developed across said resistor.

5. In combination with a tube having at least a cathode, control grid, output electrode and an auxiliary electrode positioned to receive more electrons when the control grid becomes negative, a wave input circuit coupled to the control grid and cathode, a Wave output circuit coupled to the output electrode, a load circuit independent of the output circuit connected to the auxiliary electrode, said load consisting of a resistor whose resistance value is relatively low with respect to the resistance value of said output circuit, and a reactive coupling between the auxiliary electrode and control grid, said reactive coupling being a condenser external of said tube.

6. In combination in an amplifier of high frequency waves, a tube having a positive transconductance section and a negative transcon ductance section, a wave input circuit connected to the input electrodes of the positive section, said input circuit comprising an inductance in series with a variable resistor, a fixed resistor being shunted across the inductance, a Wave output circuit connected to the output electrodes of the positive section, a resistive load connected to the output electrodes of the'negative section, a capacitance coupling the output electrodes of the negative section and the input electrodes of the positive section, said positive section input electrodes additionally acting as the negative section input electrodes, said variable resistor being adjustable in resistance value to select the operating frequency of said amplifier.

EDWARD W. HERQLD. 

